1. Field of the Invention
This invention relates to network protector relays used to control circuit breakers connecting feeders to low-voltage secondary power distribution networks, and more particularly, to a network protector relay which responds to a close command.
2. Background Information
Low-voltage secondary power distribution networks consist of interlaced loops or grids supplied by two or more sources of power, in order that the loss of any one source will not result in an interruption of power. Such networks provide the highest possible level of reliability with conventional power distribution and are, normally, used to supply high-density load areas, such as a section of a city, a large building or an industrial site. Each source is a medium voltage feeder supplying the network and consisting of a switch, a transformer and a network protector. The network protector consists of a circuit breaker and a control relay. The control relay senses the transformer voltages, the network voltages and the line currents, and executes algorithms to initiate breaker tripping or reclosing action. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the primary feeder.
Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; and 5,844,781.
Traditionally, network protector relays were electromechanical devices, which tripped the circuit breaker open upon detection of power flow in the reverse direction. Such relays were provided with a recloser, which reclosed the circuit breaker following a trip when conditions were favorable for forward current flow upon reclosing of the breaker. The electromechanical network protector relays are being replaced. One type of electronic network protector relay mimics the action of the electromechanical relay by calculating power flow.
Another type of electronic network protector relay uses sequence voltages and currents to determine the direction of current flow for making tripping decisions. Sequence analysis, upon which such relays are based, generates three vector sets to represent a three-phase voltage or current: (1) a positive sequence vector, (2) a negative sequence vector, and (3) a zero sequence vector. U.S. Pat. No. 3,947,728 discloses a sequence based network protector relay which uses the positive sequence current and positive sequence voltage vectors to make trip decisions.
More recently, digital sequence based network protector relays have been utilized which periodically sample (e.g., 8, 16, 32 times per cycle) the current and voltages.
FIG. 1 illustrates a secondary power distribution network system 1, which includes a low voltage grid 3 servicing various loads 5. The secondary network bus or grid 3 is energized by multiple sources in the form of feeders 7a,7b,7c,7d. Feeders 7a and 7b are supplied directly from substations 9a and 9b, respectively. Each of the feeders 7a-7d respectively includes a feeder bus 11a-11d, a switch 13a-13d, a feeder transformer 15a-15d, and a network protector 17a-17d. The secondary network system 1 and its components are three-phase wye or delta connected, although FIG. 1 shows these as a single line for clarity. Each of the network protectors 17a-17d includes network protector circuit breakers 19a-19d and network protector control relays 21a-21d, respectively.
As disclosed in U.S. Pat. No. 5,822,165, which is incorporated by reference herein, the control relays 21a-21d each include a microcontroller-based circuit (not shown) which monitors the network phase to neutral voltages Vn (e.g., Van, Vbn, Vcn), the transformer phase to neutral voltages Vt (e.g., Vat, Vbt, Vct), and the feeder currents I (e.g., Ia, lb, Ic).
Typically, control relays include a communication module for communication with a remote station over a communication network (hereinafter referred to as a xe2x80x9ccommunication subsystemxe2x80x9d in order to avoid confusion with the secondary network bus 3). For example, one or more MPCV control relays, which are marketed by Cutler-Hammer, may be connected to the communication subsystem (e.g., PowerNet or IMPACC Series III as marketed by Cutler-Hammer) to allow remote access to protector measurement data of interest. In turn, the control relays perform breaker trip and reclose functions.
In addition, remote tripping, or more specifically xe2x80x9cremote open and block openxe2x80x9d control, has been implemented over the communication subsystem. This allows users, such as electric utility maintenance personnel, to remotely open the circuit breaker. See, for example, U.S. Pat. No. 5,936,817. Otherwise, the circuit breaker would remain closed based on the programmed protection parameters of the network protector relay. Those protection parameters open, and continue to block the network protector relay from attempting an automatic reclosure, until a subsequent xe2x80x9cremovexe2x80x9d block open command is issued. This control is inherently a safe operation, even from a remote location, since energy sources(s) (e.g., transformer(s)) are removed from the network.
Normally, network protector relays, when tripped (open), begin an automatic reclose mode. If the system conditions are correct, then the relay commands a reclosure of the circuit breaker. Specifically, the relay senses the phasing voltage (i.e., the transformer to network difference voltage, which is the voltage across the circuit breaker terminals) relative to the network voltage. In order to generate a reclose command, as disclosed in U.S. Pat. No. 5,822,165, the phasing voltage measurement is compared with various setpoint characteristics. As shown in FIG. 2, the voltage V1p (shown as Vp1 in FIG. 6 of U.S. Pat. No. 5,822,165) has: (1) a magnitude greater than a user-defined 0xc2x0 master threshold value (Vm); and (2) an angle which is between user-adjustable blinder angles 79,81. Angle 79 is in the negative q, positive d quadrant (II) near the negative q axis, while angle 81 is in the positive q, positive d quadrant (I) near the positive d axis. Also, a circular reclose line 83 is employed which allows circuit breaker reclosing at lighter network loads (e.g., in region 85) than if not provided.
Although the prior art shows various types of reclose algorithms, there is room for improvement.
The present invention provides a safe implementation of a remotely communicated xe2x80x9cclosexe2x80x9d command (e.g., from a personal computer over a communication subsystem) to a network protector control relay. In accordance with the invention, a qualified remote close command is provided.
As one aspect of the invention, a network protector relay for a circuit breaker includes means for providing a positive sequence network voltage from polyphase network voltages on a network bus; means for providing a positive sequence phasing voltage from the polyphase network voltages on the network bus and polyphase feeder voltages on a feeder bus; means for automatically reclosing the circuit breaker connected between the polyphase feeder bus and the polyphase network bus in response to a function of a plurality of setpoints and the positive sequence phasing voltage which indicates a first flow of power from the polyphase feeder bus to the polyphase network bus; means for receiving a close command; and means for temporarily changing at least one of the setpoints in response to the received close command, in order to conditionally close the circuit breaker for a second flow of power from the polyphase feeder bus to the polyphase network bus, with the second flow of power being less than the first flow of power.
Preferably, the at least one of the setpoints includes a master characteristic setpoint and the means for temporarily changing includes means for temporarily reducing the master characteristic setpoint. The master characteristic setpoint has a predetermined value, and the means for temporarily changing further includes means for restoring the master characteristic setpoint to the predetermined value after a predetermined time.
As another refinement, the at least one of the setpoints includes a master characteristic setpoint, the means for temporarily changing includes means for temporarily reducing the master characteristic setpoint, and the means for automatically reclosing includes means for comparing the positive sequence phasing voltage to the reduced master characteristic setpoint.
As yet another refinement, the means for providing a positive sequence phasing voltage includes means for providing a corresponding phase angle with the positive sequence phasing voltage, and the means for automatically reclosing includes means for comparing the phase angle to a predetermined angle.
As another aspect of the invention, a network protector relay for controlling a circuit breaker includes means for providing a positive sequence network voltage from polyphase network voltages on the network bus; means for providing a positive sequence phasing voltage from the polyphase network voltages on the network bus and polyphase feeder voltages on the feeder bus; means for automatically reclosing the circuit breaker connected between the polyphase feeder bus and the polyphase network bus in response to a function of a plurality of setpoints and the positive sequence phasing voltage which indicates a first flow of power from the polyphase feeder bus to the polyphase network bus; means for receiving a remote close command; and means for qualifying the remote close command and conditionally initiating closure of the circuit breaker.